1. Field of the Invention
The present invention generally relates to a variable delivery type fuel supply apparatus which can be employed, for example, in a cylinder injection type internal combustion engine. More particularly, the present invention is concerned with a variable delivery type fuel supply apparatus which includes an electromagnetic valve disposed in an oil relief passage and adapted to be opened for a predetermined time during a delivery or discharge stroke of a fuel pump for thereby controlling a delivery or discharge quantity of a fuel.
2. Description of Related Art
For better understanding of the concept underlying the present invention, description will first be made of a conventional variable delivery type fuel supply apparatus known heretofore. FIG. 9 of the accompanying drawings shows a circuit diagram of a fuel supply system which includes a variable delivery type fuel supply apparatus 100. Referring to the figure, the variable delivery type fuel supply apparatus 100 is comprised of a low-pressure damper 2 disposed in association with a low-pressure fuel suction passage 1 for absorbing pulsation of a low-pressure fuel flow (i.e., fuel flow of a low pressure), a high-pressure fuel pump 4 for pressurizing a low-pressure fuel fed from the low-pressure damper 2 for discharging pressurized fuel into a high-pressure fuel delivery passage 3, a fuel pressure holding valve 5 for holding a pressure of the fuel flowing through the high-pressure fuel delivery passage 3 under a high pressure, an oil relief passage 6 provided for interconnecting a suction port of the fuel pump 4 and a pressurizing chamber thereof, an electromagnetic valve (which may also be referred to as the solenoid valve) 7 disposed in the oil relief passage 6 and adapted to be opened for a predetermined time during the discharge stroke of the fuel pump 4 for adjusting or regulating the fuel discharge quantity of the fuel pump 4, and a control unit 8 for controlling the valve open timing of the electromagnetic valve 7. The fuel pump 4 includes a suction valve 4a and a discharge valve 4b. 
On the other hand, as the peripheral components of the variable delivery type fuel supply apparatus 100, there are provided a fuel tank 10, a low-pressure fuel pump 11 disposed within the fuel tank 10, a low-pressure regulator 12 disposed in the low-pressure fuel suction passage 1 through which the low-pressure fuel discharged from the low-pressure fuel pump 11 flows, for stabilizing the pressure of the fuel flow, a relief valve 15 disposed in a drain pipe 14 branched from the high-pressure fuel delivery passage 3 at a branching portion 13, and fuel injection valves 18 mounted on a delivery pipe 17 connected to the high-pressure fuel delivery passage 3, and a filter 19 disposed in the drain pipe 14 at an appropriate location thereof.
In operation of the variable delivery type fuel supply apparatus 100 implemented in the structure described above, the suction valve 4a of the fuel pump 4 is opened during the suction stroke to suck the fuel in the pressurizing chamber, while in the discharge stroke, the discharge valve 4b is opened, whereby the fuel within the pressurizing chamber is discharged into the delivery pipe 17 equipped with the fuel injection valves 18. The oil relief passage 6 is provided across the suction valve 4a of the fuel pump 4 for interconnecting the suction port of the high-pressure fuel pump 4 and the pressurizing chamber thereof. The solenoid or electromagnetic valve 7 disposed in the oil relief passage 6 serves for adjusting or regulating the discharge quantity of the fuel pump 4 by opening for a predetermined time in the course of discharge stroke of the fuel pump 4. The control unit 8 is in charge of controlling the valve open timing of the electromagnetic valve 7.
FIG. 10 is a timing chart for illustrating the control or driving signal supplied to the electromagnetic valve 7 from the control unit 8 together with suction/discharge strokes of the fuel pump 4. Referring to FIG. 10, the plunger lift is taken along the ordinate at a top row, in which hatched areas indicate, respectively, the amounts or quantities of the fuel discharged from the fuel pump 4. Further taken along the ordinate at a bottom row is a waveform of an electric current applied for driving the electromagnetic valve 7 which is implemented as a normally closed valve adapted to be opened, i.e., turned on, when it is electrically energized. In the conventional variable delivery type fuel supply apparatus 100, the time point for terminating the electrical energization of the valve 7 (i.e., time point for closing or turning off the valve 7) is fixed at a predetermined timing whereas the energization starting time point (valve open or turn-on time point) is set variable relative to the suction/discharge stroke of the fuel pump 4 for the purpose of controlling the discharge quantity thereof. More specifically, in the case of the example illustrated in FIG. 10, the electrical energization terminating time point (valve close time point) is fixedly set at the end of the discharge stroke (or at the start of the suction stroke, to say in another way), whereas the electrical energization starting time point (valve open time point) is controlled variably during the discharge stroke).
In general, the open/close operation of the electromagnetic or solenoid valve 7 triggered in response to a control signal issued from the control unit 8 is accompanied with a time lag more or less in practical applications. Accordingly, in the conventional variable delivery type fuel supply apparatus, the time lag mentioned above, i.e., delay of response of the electromagnetic valve 7, is arithmetically estimated by the control unit 8 in advance, and the electromagnetic valve 7 is driven or controlled by taking into account the estimated or expected time lag. In this conjunction, it is however noted that such time lag (delay of response) will vary in dependence on changes of the supply voltage, the ambient temperature and other factors, e.g. lowering of the supply voltage, rise of the ambient temperature and/or the like. For this reason, fuel control of the discharge quantity is likely to become nonuniform, giving rise to a problem that variation tends to occur in the fuel pressure within the delivery pipe 17 and hence in the fuel quantity injected through the fuel injection valves 18. In order to cope with this problem, a much complicated control procedure has to be adopted in the conventional variable delivery type fuel supply apparatus.
In the light of the state of the art described above, it is an object of the present invention to provide a variable delivery type fuel supply apparatus which is capable of reducing or suppressing variation in the fuel pressure within the delivery pipe and hence variation of the fuel quantity injected through the fuel injection valve and which apparatus allows the control method to be simplified.
In view of the above and other objects which will become apparent as the description proceeds, there is provided according to a general aspect of the present invention a variable delivery type fuel supply apparatus which is comprised of a fuel pump including a suction valve adapted to be opened during a suction stroke for sucking a fuel in a pressurizing chamber in the course of reciprocation of a plunger within a cylinder and a discharge valve adapted to be opened during a discharge stroke for discharging the fuel from the pressurizing chamber into a high-pressure fuel delivery passage of an internal combustion engine equipped with fuel injection valves in the course of reciprocation of the plunger within the cylinder, an oil relief passage provided between a suction port of the fuel pump and the pressurizing chamber across the suction valve of the fuel pump so as to interconnect the suction port and the pressurizing chamber, an electromagnetic valve disposed in the oil relief passage and adapted to be opened for a predetermined time during the discharge stroke of the fuel pump for thereby controlling a fuel discharge quantity of the fuel pump, and a control unit for controlling timing at which the electromagnetic valve is opened, wherein the control unit is so designed as to control open/close operations of the electromagnetic valve such that a time point for starting electrical energization of the electromagnetic valve is fixedly set at a predetermined time point relative to the suction/discharge stroke of the fuel pump while allowing a time point for terminating the electrical energization to be variable, to thereby control the fuel discharge quantity of the fuel pump.
By virtue of the arrangement described above, variation of the fuel pressure in the delivery pipe and hence variation of the fuel quantity injected through the fuel injection valve can effectively be suppressed. Further, the response time of the electromagnetic valve is rendered less susceptible to the influence of lowering of the supply voltage, rise of the ambient temperature and the like factors. To say in another way, much stabilized response behavior or performance of the electromagnetic valve can be ensured substantially under any conditions. In addition, because the delay of response remains essentially constant independent of the conditions such as mentioned above, the control method can be much simplified.
In a preferred mode for carrying out the invention, the electromagnetic valve may be implemented as a normally closed valve which is designed to be opened when electrically energized.
With the arrangement described above, the variable delivery type fuel supply apparatus according to the invention can be rendered more insusceptible to the influence of variation of the response time, whereby further enhanced stabilization can be ensured for the response performance of the variable delivery type fuel supply apparatus.
In another mode for carrying out the invention, the electromagnetic valve may preferably be comprised of a valve element which is subjected to a high pressure in a direction perpendicular to an axial direction of a valve closing spring.
Owing to the arrangement mentioned above, the response delay behavior of the electromagnetic valve can be stabilized, whereby the response performance of the variable delivery type fuel supply apparatus can further be stabilized and improved. Besides, the time duration of electrical energization of the electromagnetic valve can be shortened relative to that of the suction/discharge stroke, whereby the coil temperature rise suppressing effect as well as the power consumption reducing effect can significantly be enhanced.
In yet another mode for carrying out the invention, the control unit should preferably be so designed that an electromagnetic valve driving current is increased to a high level immediately after electrical energization of the valve is started while the electromagnetic valve driving current is held at a low level after lapse of a predetermined time since the start of the electrical energization.
Owing to the feature described above, the electric energy supplied to the electromagnetic valve can be reduced, whereby not only the temperature rise of the solenoid or coil can effectively be suppressed but also the power consumption can positively be reduced, to further advantageous effects.
The above and other objects, features and attendant advantages of the present invention will more easily be understood by reading the following description of the preferred embodiments thereof taken, only by way of example, in conjunction with the accompanying drawings.